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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.

1	ChangeLog:
2		Started by Ingo Molnar <mingo[AT]redhat[DOT]com>
3		Update by Max Krasnyansky <maxk[AT]qualcomm[DOT]com>
4	
5	SMP IRQ affinity
6	
7	/proc/irq/IRQ#/smp_affinity specifies which target CPUs are permitted
8	for a given IRQ source. It's a bitmask of allowed CPUs. It's not allowed
9	to turn off all CPUs, and if an IRQ controller does not support IRQ
10	affinity then the value will not change from the default 0xffffffff.
11	
12	/proc/irq/default_smp_affinity specifies default affinity mask that applies
13	to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
14	will be set to the default mask. It can then be changed as described above.
15	Default mask is 0xffffffff.
16	
17	Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
18	it to CPU4-7 (this is an 8-CPU SMP box):
19	
20	[root@moon 44]# cd /proc/irq/44
21	[root@moon 44]# cat smp_affinity
22	ffffffff
23	
24	[root@moon 44]# echo 0f > smp_affinity
25	[root@moon 44]# cat smp_affinity
26	0000000f
27	[root@moon 44]# ping -f h
28	PING hell (195.4.7.3): 56 data bytes
29	...
30	--- hell ping statistics ---
31	6029 packets transmitted, 6027 packets received, 0% packet loss
32	round-trip min/avg/max = 0.1/0.1/0.4 ms
33	[root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
34	           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
35	 44:       1068       1785       1785       1783         0          0           0         0    IO-APIC-level  eth1
36	
37	As can be seen from the line above IRQ44 was delivered only to the first four
38	processors (0-3).
39	Now lets restrict that IRQ to CPU(4-7).
40	
41	[root@moon 44]# echo f0 > smp_affinity
42	[root@moon 44]# cat smp_affinity
43	000000f0
44	[root@moon 44]# ping -f h
45	PING hell (195.4.7.3): 56 data bytes
46	..
47	--- hell ping statistics ---
48	2779 packets transmitted, 2777 packets received, 0% packet loss
49	round-trip min/avg/max = 0.1/0.5/585.4 ms
50	[root@moon 44]# cat /proc/interrupts |  'CPU\|44:'
51	           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
52	 44:       1068       1785       1785       1783      1784       1069        1070       1069   IO-APIC-level  eth1
53	
54	This time around IRQ44 was delivered only to the last four processors.
55	i.e counters for the CPU0-3 did not change.
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